This invention relates to improved ventilated insulated roofing systems. More particularly, this invention concerns roofing systems which employ as the insulation medium, or as a part of the insulation medium, a layer of normally low-permeance cellular plastic material such as foamed polystyrene, which systems are improved with respect to the ability to vent or release moisture entrapped within.
Insulation boards comprised of a normally low-permeance, that is, usually less than about 3 to 5 perms, cellular synthetic resinous material such as foamed or expanded polystyrene, polyurethane, etc., have been widely employed as a thermal insulation medium in roofing systems, especially systems wherein the various components are installed or "built-up" at the job site. For instance, in U.S. Pat. No. 3,094,477 there is described a method of preparing a built-up roof wherein a board of foamed polystyrene is adhered to the structural deck and several layers of a bitumen and felt are applied above to provide the waterproofing membrane. One serious problem associated with the use of such low-permeance boards in the roofing is the entrappment of moisture vapor beneath the impermeable waterproofing. Such moisture vapor, especially on hot days, can cause the formation of bubbles and subsequent leaks in the bituminous waterproofing membrane.
The problem of moisture entrappment can be of all the more concern in roofing systems such as that described in U.S. Pat. No. 3,619,961 to Sterret et al wherein a layer of the foamed insulation board is employed in conjunction with a layer, more often a number of layers, of a moisture-bearing material such as lightweight insulating concrete. The foamed board enhances the thermal insulation of the system without adding appreciably to the weight of the roof. The insulating concretes are generally mixtures of Portland cement and a lightweight aggregate such as expanded vermiculite or perlite, fly ash, etc. Whereas, the water to cement ratio for structural Portland cement concretes lies generally between 0.4 and 0.7, a typical water to cement ratio for such lightweight insulating concretes ranges in excess of 2. This excess mix water employed to render the material sufficiently fluid for placement can easily become entrapped beneath the waterproofing in these systems by the impermeable foam. In the Sterrett et al patent, the problem of venting such entrapped moisture is solved by grooving a number of the lateral surfaces of the plastic foam insulation board which act to convey the moisture around the board and eventually outside the roofing system. In another previous system wherein the foamed insulation board has been "sandwiched" between two layers of lightweight insulating concrete, holes have been provided through the layer of foam insulation and the fluid concrete from the upper layer allowed to flow into and fill up the openings. In this system, the "bridges" of concrete formed in the foam layer serve chiefly to key the upper layer of concrete to the lower layer and as passageways for moisture since the concrete itself is moisture permeable.